Backflow Preventer

ABSTRACT

A reduced pressure zone backflow preventer comprising a body  12  entirely made of plastic material, formed with an inlet port ( 14 ) accommodating an inlet check valve assembly  120  and an outlet port accommodating an outlet check valve assembly  122 . The check valve assemblies extend along a coaxial axis coinciding with a longitudinal axis of the body. A central space  60  formed between the check valves is accessible through a service opening in the body. A relief valve assembly  70  is received within the central space and a drain port extends from the space. The relief assembly is responsive to pressure differential between the inlet port and the space by a flow passage extending between the inlet port and an upper side of a pressure diaphragm of the relief valve assembly.

FIELD OF THE INVENTION

The present invention relates to fluid backflow preventers and more particularly it is concerned with Reduced Pressure Zone backflow preventers, often referred to as RPZ. Even more specifically, the present invention is concerned with structural features of an RPZ.

BACKGROUND OF THE INVENTION

Backflow preventers are well known and are typically used for allowing flow of water (or other fluid) from a source (such as a municipal water supply) to an end user (consumer). A backflow preventer is used to prevent flow of the fluid (e.g. water) in the reverse direction (e.g., for avoiding contamination of a municipal potable water supply, by contaminated water, chemicals, fertilizers, and pesticides and the like which may be supplied through an automatic dosing device, etc.). Such systems are often obligatory under different municipal and other codes and requirements.

A number of configurations are known in the art for avoiding backflow including vacuum breaker systems, Reduced Pressure Zone backflow preventers—RPZs, and double check valve systems. In a double check valve system, the fluid, in normal use, flows through a first check valve, into a zone between the two check valves and then flows through a second check valve. The check valves are of either a normally closed or a normally open type, depending on the configuration of the device. Further provided there is a relief valve in flow communication with the zone for discharge of water (or other fluid), in case of reverse flow, due to change in the pressure system and malfunction of the device. Such a relief valve is of either a normally closed or a normally open type, depending on the configuration of the device.

During normal use, the zone is maintained at a pressure lower than the inlet pressure. If there is a pressure at the outlet port which is not at least a predetermined amount lower than the pressure at the inlet port, the check valves will close, thus preventing backflow. According to another arrangement, the first and second check valves are normally closed and displace into their respective open position under fluid pressure.

If abnormal conditions arise, in which there is no flow, or reverse flow, and the second check valve is in a failed state (e.g. is arrested in an open/closed position), the differential release valve will open and discharge liquid to the environment, so as to avoid backflow to the source supply.

Backflow preventer valves must often withstand high pressures (such as about 150 psi or more), thus requiring the use of a massive and reinforced body structure.

In some prior art backflow preventers the inlet and outlet ports are non-parallel and/or non-coaxial and/or provide flow in opposite directions with respect to one another resulting in the need for additional fittings for installation in a so called ‘in-line’ installation (which is common in point-of-use applications), undesirably adding to the cost of design, installation, maintenance and the like, as well as potentially increasing the size or volume of the device as installed.

Some prior art backflow preventers are configured such that servicing and maintenance is performed through different openings of the body of the device. Such configurations are believed to be inconvenient in many situations, in particular at point-of-use installations, where the available space for accommodating access to the device is restricted.

In several prior art backflow prevention devices, at least some of the components are received within a separate or enclosure, thus requiring installation of bolts through flanges or other coupling devices in order to achieve the desired total assembly. Such coupling devices typically add to the volume, length and/or weight of the total assembly, as well as to the general labor and costs involved.

Furthermore, many backflow preventers require for installation or removal of some or all components special tools and devices which generally require hand and/or power tools (wrenches, screwdrivers, nut-drivers etc. and in some lo cases specially designed tools), typically involving inserting or removing bolts, nuts, screws and the like.

This necessarily leads to relatively high costs of fabrication, assembly, maintenance and repair, and is further not suitable at tight space installations, as mentioned hereinabove. Prior art preventer backflow devices are disclosed, for example, in the following list of U.S. Pat. Nos. 4,489,746, 6,021,805, 6,346,736, 6,513,543, 6,546,946, and further in Patent Applications US 2002/0062868A1, EP 0964104A2 and WO 02/21029A1.

It is an object of the present invention to provide a reduced pressure zone backflow prevention device in which substantially all components, at least the first and second check valves and the relief valve, are enclosed within a single housing without the need for a coupling device and are easily accessibly, wherein a plurality of its components, and in particular both check valves, can be accessed for repair or maintenance from a single location.

It is a further object of the invention to provide a backflow prevention device with minimal components and where hardly any tools are required for fabrication, assembly, servicing, and maintenance.

It is still an object of the invention to provide a backflow prevention device with inlet and outlet openings which are parallel, and still preferably coaxial with on another and defining flow in the same direction with respect to one another.

According to the present invention there is provided a reduced pressure zone backflow preventer, herein the specification and claims referred to in short as an ‘RPZBP’ wherein the above disadvantages are substantially reduced or overcome and where the above advantages are exploited.

SUMMARY OF THE INVENTION

A backflow prevention device according to the present invention offers the above features and other objects which are achieved by an assembly wherein significant components thereof are made of elastomeric materials.

Accordingly, the invention calls for a reduced pressure zone backflow preventer (RPZBP) comprising a body made of plastic material formed with an inlet port accommodating an inlet check valve assembly and an outlet port accommodating an outlet check valve assembly, said check valves extending along a coaxial axis coinciding with a longitudinal axis of the body, and a central space formed between said check valves accessible through an opening, with a relief valve assembly received within the central space and a drain port extending from said space; said relief assembly is responsive to pressure differential between the inlet port and the space by a flow passage extending between the inlet port and upper side of a pressure diaphragm of the relief valve assembly.

The space further accommodates a retention member fixedly receivable within the space and extending between the check valves so as to bear axial loads applied by said check valves, though without transferring axial loads between said check valves.

The structure of the RPZBP is such that the inlet check valve assembly, the outlet check valve assembly, the relief valve assembly and the retention member are slidingly introduced through the service opening and are retained at their respective locations without the need for any fixtures such as screws and bolts, etc. important note is made to the fact that the body is a uniform solid structure made of plastic material, optionally of composite material for reinforcement thereof.

The central space within the body has a longitudinal axis intersecting the longitudinal axis of the body at an essentially right angle. Typically, the body assumes a shape of two cylindrical bodies intersected at a right angle.

The RPZBP according to the present invention has many unique features which are possible owing to the structure thereof, stating for example the following:

-   -   Both the inlet check valve assembly and the outlet check valve         assembly are modular, easy replaceable and designed for         foolproof assembly so as to prevent reverse assembly;     -   Essentially no tools are required for assembly/disassembly and         servicing of the device, apart for a tool for opening the         service opening, such as a screwdriver;     -   The entire device, apart for some screws and springs is made of         molded (injected) plastic material, optionally reinforced by         composite material technology;     -   Several test cocks are provide for calibration and testing, as         may be required under different codes and standards;     -   In the inadvertent case of failure of the inlet check valve,         even if the piston stem is arrested at a position projecting         into the space, the retention member can nevertheless be removed         from the space for servicing;     -   The retention member is biased in its location within the space         by a spring of the relief valve assembly, whilst the retention         member supports and guides the spring and stem member.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carried out in practice, an embodiment will now be described, by way of a non-limiting example only, with reference to the accompanying drawings, in which:

FIG. 1 is an back/top isometric view of the RPZBP according to the prevent invention;

FIG. 2 is a front/top exploded isometric view of the RPZBP seen in FIG. 1;

FIG. 3A is sectional view along a longitudinal axis of the RPZBP in its normally closed position;

FIG. 3B is a top view of the body of the RPZBP with the cover removed;

FIGS. 4 are directed to the inlet check valve assembly, wherein:

FIG. 4A is an exploded isometric view of the inlet check valve assembly;

FIG. 4B is an isometric view of the inlet check valve assembly, in its normally closed position;

FIG. 4C is an isometric sectional view of the inlet check valve assembly of FIG. 4B;

FIG. 4D is an isometric view of the inlet check valve assembly, in its open position; and

FIG. 4E is an isometric sectional view of the inlet check valve assembly of FIG. 4D;

FIG. 5 are directed to the outlet check valve assembly, wherein:

FIG. 5A is an isometric view of the outlet check valve assembly, in its normally closed position;

FIG. 5B is an isometric sectional view of the outlet check valve assembly of FIG. 5A;

FIG. 5C is an isometric view of the outlet check valve assembly, in its open position; and

FIG. 5D is an isometric sectional view of the outlet check valve assembly of FIG. 5C;

FIG. 6 are directed to the retention element, wherein:

FIG. 6A is a top isometric view of the retention element;

FIG. 6B is a bottom isometric view of the retention element;

FIG. 6C is a side view of the retention element;

FIG. 6D is a longitudinal section of the retention element along line IV-IV in FIG. 6C;

FIG. 7 is sectional view along a longitudinal axis of the RPZBP in its open position;

FIG. 8 is longitudinal sectional view of an RPZBP according to a modification thereof, also fitted with a discharge spout;

FIG. 9A is an isometric view of a straight connector used in the RPZBP seen in FIG. 8;

FIG. 9B is an isometric view of an elbowed connector used in the RPZBP seen in FIG. 8; and.

FIG. 10 is an enlargement of the portion marked X in FIG. 4E, illustrating a detail of a seal ring of a valve piston.

DETAILED DESCRIPTION OF THE INVENTION

Attention is first directed to FIG. 1 of the drawings for general familiarization with the backflow preventer device according to the present invention, formed with an elongate cylindric body 12 made of rigid plastic material extending between an inlet end with an inlet port 14 and an outlet end with an outlet port 16, coaxially aligned. The body is a uniform rigid solid structure made of plastic material, optionally of composite material for reinforcement thereof.

The body 12 further comprises an upright extending cylindrical portion 18 extending at a right angle with respect to the longitudinal axis of the body 12 and formed with an opening (29 in FIG. 2) sealingly covered by a cover 22 fixedly secured in place by a plurality of screws 24.

Three test cocks 28A (being in flow communication with the inlet port through aperture 59; see FIG. 3A), 28B (being in flow communication with the central space 60 through aperture 61; see FIG. 3A) and 28C (being in flow communication with the outlet port through aperture 63; see FIG. 3A) are fixedly attached to the housing typically in the form of ball valves fitted with a screw like actuator 30 (see FIG. 1), to prevent unintentional opening thereof.

Clear indications in the form of arrows 32A, 32B, 32C and 32D are marked at various locations of the housing to provide correct guidance for direction of installation of the device.

With further reference made now also to FIGS. 2 and 3, attention is directed to the housing 12 and as can be seen the inlet port 14 and the outlet port 16 are fitted with an annular flanged portion 36 and 38, respectively for articulating thereto of an end piece which may assume different shapes, depending on the installation configuration. In the particular example of FIGS. 1 to 3, the end pieces 40 are identical and are in the form of a straight threaded connecting portion formed with an annular flange 42 clampingly secured against the respective flanged portion of the housing (36 and 38, respectively) by a partitioned bracing collar comprising an upper band 46 and a lower band 48 screw coupled by a pair of screws 50.

The end pieces 40 (see also FIG. 9A) are formed with a cylindric portion 52 snugly received within an end portion of each of the inlet port 14 and the outlet port 16 with an annular groove receiving a sealing 0-ring 54.

As can further be seen (best in FIGS. 2 and 3B), the housing 12 is fitted with a central space 60 accessible through opening 20 which space is formed with two upright extending wall portions 63 extending between a pair of grooves 64 and 66 (both of which are only partially seen in FIG. 2). The walls 63 provide a double-walled portion for reinforcement of the body structure at the enlargement of the central space zone 60 and further said walls 64 have a role in supporting a retention member 68 and ensuring its correct position in a full-proof manner as will be explained hereinafter. Another important role of the retention member 68 and the grooves 64 and 66 is to bear axial loads applied by check valves of the system, as will become apparent hereinafter.

Received within the central space 60 is a relief valve assembly generally designated 70 (FIG. 2) and comprising a resilient diaphragm 72 mounted between two rigid plastic discs 74 and 76 screw coupled to one another with a stem member 78 downwardly extending and supporting at its bottom end a sealing ring 80 extending above a guiding portion 82 sized so as to ensure smooth sliding within a tubular section 86 at a discharge outlet 88 extending from the space 60, which outlet is normally open but spontaneously seals under the influence of water flow through the device, by sealing ring 80 bearing against sealing seat 90 (FIGS. 3 and 7). Projecting upwardly from the top disc 74 there is a cylindric section 92 fitted with an 0-ring 94 sealingly and snugly received within an annular receptacle 96 formed in the cover 22 (FIG. 3A) slidingly supporting the relief valve assembly 70.

Mounted on stem 78 there is a coiled spring 102 which at the assembled position is supported at its upper end by an annular support wall 106 of the lower disc 76 and bearing at its lower end over an annular support portion 110 of the retention member 68 (FIGS. 2, 3 and 6A).

As can best be seen in FIGS. 3 and 7, the cover 22 is formed with an aperture 114 for airing the space 96 such that the opposite ends of the relief valve assembly 70, which are essentially of same diameter, are under pressure equilibrium, as both are aired/vented.

Introduced through the space 60 and snugly received within the housing 12 there is an inlet check valve assembly 120 and an outlet check valve assembly 122, the structure and operation thereof will be explained in detail with reference to FIGS. 4 and 5, respectively. It is however noted that both the check valve assemblies 120 and 122 are modular capsule-like elements differentiated from one another by unique forms of their facing tabbed ends 130 and 132, respectively, to ensure fall-proof assembly of the device. Each of the check valve assemblies comprises an 0-ring 136 for sealing engagement against the cylindrical walls of the cylindrical receiving portions, respectively, within the housing 12.

It is appreciated that insertion and extraction of the inlet check valve assembly 120 and the outlet check valve assembly 122 obviates the use of any tools as these assemblies are merely introduced through the central space 60 (prior to introducing the retention member 68) whilst extraction thereof is facilitated either by pushing each check valve assembly through its respective port towards the central space 60 or by a leverage tool (any available tool) applying force on the tabbed wall portion 130 and 132, respectively.

Turning now to FIGS. 4 and 5, there are illustrated the inlet check valve assembly 120 and the outlet check valve assembly 122, respectively. Noting the similarity between the check valve assemblies, like elements at the outlet check valve assembly will be designated same reference numbers with a prime (′) indication. Similarity of the components of the check valve assemblies substantially reduces in costs involved in manufacturing and maintenance of a backflow prevention device in accordance with the present invention.

As already mentioned, the check valve assemblies are in the form of cylindrical capsules 140; 140′, with their facing ends (i.e. ends facing the central space 60 within the body) formed with a projecting tab 130; 132. In accordance with a typical embodiment, there is an inscription at said facing ends reading “inlet” and “outlet”, respectively. The arrangement is such that only upon correct and complete insertion of the inlet check valve assembly 120 and the outlet valve assembly 122, the retention member 68 may then be inserted and seated within the space 60, as will become apparent herein later with further reference to FIGS. 6A to 6B.

The check valve assemblies slidingly accommodate a sphere-like piston body 150; 150′ fitted with a stem member 152; 152′ supporting a coiled spring 154; 154′ biasing the piston member into sealing engagement against a sealing seat surface 158; 158′ (position seen in FIGS. 4B, 4C and 5A, 5B, respectively) by means of an elastomeric seal ring 162; 162′ mounted on the piston body 150; 150′.

The assembly is packed into the housing 140; 140′ by a cover member 166; 166′ snappingly engaged with the housing 140 and fitted with a sleeve portion 168; 168′ (best seen in FIG. 4A) slidingly receiving the stem member 152; 152′ and supporting the coiled spring 154; 154′.

The arrangement is such that in their normal position the inlet check valve assembly 120 is closed and will open only upon water pressure at its inlet side to facilitate flow in the predetermined position, i.e. from the inlet port 14 towards the central space 60, as indicated by the arrows in FIGS. 1 to 3, and the outlet check valve assembly 122 is normally closed and will facilitate water flow there-through only upon water pressure at the space 60 to allow water flow in the direction from the central space 60 towards the outlet port 16 as indicated.

Turning now to FIGS. 6A to 6D, there is illustrated the retention member 68, also made of a rigid plastic material and comprising a central path 180 which in the assembled position is coaxial with the longitudinal axis of the backflow preventer device 10 and coincides with the longitudinal axis of each of the inlet check valve assembly 120 and the outlet check valve assembly 122. The retention member 68 further comprises a top opening 184 (FIG. 6A) and a bottom opening 186 (FIG. 6B) aligned with one another and intersecting the longitudinal axis 180 at a right angle (i.e. coincides with the longitudinal axis of the cylindrical portion 18 of housing 12). As seen in FIGS. 6A, 6B and 6D, the bottom portion of the retention member (facing in assembly the inlet check valve assembly 120) comprises a recess 190, the purpose of which will become apparent hereinafter.

Further noticed, the central path 180 of the retention member 18 has a conical cross-section (FIG. 6D), corresponding with the respective ends of the adjoining check valve assembly 120 and the outlet check valve assembly 122, so as to ensure smooth flow therethrough and to avoid turbulent currencies and pressure shocks through the device.

Further noticed, the retention member 68 is fitted with two circumferential ribs 194 and 196, which ribs are not equally spaced from the respective ends of the retention member 68. This arrangement is deliberate and is to ensure that the retention member 68 is correctly introduced into the central space 60, whereby said ribs 194 and 196 are securely received within the corresponding grooves 66 and 64 formed in said substantial space 60. Furthermore, a pair of longitudinal ribs 200 extend between said circumferential ribs 194 and 196, serving both to strengthen the structure of the retention member 68 and further, to bear against the wall portions 63 in the central cavity 60 of the body 12, in the assembled position.

An arrow 197 marked on the retention member 68 indicates the flow direction through the device 10, so as to provide indications for correct and easy assembly.

When in the assembled position, top opening 184 of the retention member 68 and the bottom opening 186, axially coincide with the discharge outlet 86 of the housing 12 (FIGS. 3, 7 and 8) whereby said retention member 68 is biased into its location by coiled spring 102 of the relief valve assembly 70 and whereby the stem 78 of the relief valve assembly is free to displace through the vertical opening extending through retention member 68.

With further particular reference made to FIGS. 7 and 3, it is noted that body 12 is formed with a flow passage 210 having an inlet 214 extending at the inlet port 14 before the sealing location of the inlet check valve assembly 120, with an outlet of said passage at 216 (see FIG. 2) being in flow communication with an upper space 220 above diaphragm 72, through a passage 217 integral with the cover 22, thus forming a control chamber for the relief valve assembly 70. It is further noted that flow passage 210 is in flow communication with test cock 28A.

The arrangement is such that at a normal position of the backflow preventer 10, when there is no water flow therethrough, both the inlet check valve assembly 120 and the outlet check valve assembly are at their normally closed position. At this state, backflow pressure applied to the outlet check valve assembly 122 (represented by arrow 130 in FIG. 3A) will not open the outlet check valve assembly 122, thereby preventing backflow (upstream). However, at the inadvertent case of failure of the outlet check valve assembly 122, if for some reason it is not completely sealed and water flows into the space 60, it will then build up pressure within said space 60 and will cause displacement of the relief valve assembly 70 into its upper, open position (FIG. 3A) so as to allow discharge of the water through discharge outlet 88.

When water pressure is applied at the inlet port 14 in direction of arrow 136 in FIG. 7, the inlet check valve assembly 120 displaces into its open position to allow flow therethrough into the space 60 and further to displace the outlet check valve assembly 122 in its respective open position to facilitate flow through the device. Simultaneously, water flows through passage 210 into the control chamber 220 whereupon the relief valve assembly 70 displaces into down, closed position (FIG. 7) to prevent discharge of liquid through the discharge port 88.

As can further be seen, for example in FIG. 7, when the inlet check valve assembly 120 is in its open position, the stem 150 projects into the central path 180 formed in the retention member 68 and for that reason, the retention member is formed with the recess 190 to thereby facilitate extracting of the retention member 68 even at this condition, so as to allow servicing of the backflow preventer.

It is further noted in FIGS. 3 and 7 that the liquid passage 210 is formed as a bore extending from a forehead 234 of the flanged portion 36 of body 12, which bore is sealed by a plug 236. The arrangement is such that the plug 236 is retained in position upon coupling thereto of end piece 40 whereby the annular flange 42 tightly bears against the plug 236 and the assembly is retained in position by the partitioned bracing collars 46 and 48.

It is further seen, best in FIGS. 3 and 7, that the portion in the body 12 receiving the inlet check valve assembly 120 is formed with a first diameter 240 adjacent the space 60 and a second diameter 244 adjacent the inlet port 14, said second diameter being slightly lesser than said first diameter. This arrangement is intended to facilitate easy insertion of the inlet check valve assembly 120 through said first portion and ensure tight sealing engagement of 0-ring 136 at said second portion 244.

The embodiment of FIG. 8 is similar to the previous embodiment with the exception that the outlet end piece 250 is a right angled (“elbow”) piece secured to the outlet port 16 by a similar partitioned bracing collar as disclosed in connection with the inlet end piece 40 by means of clamping engagement of annular flange 38 of body 12 with an annular flange 254 of the outlet piece 250 through said upper band 46 and lower band 48. The end pieces 40 (FIG. 9A) and 250 (FIG. 9B) are formed with axial projections 260 provided to engage with corresponding depressions formed at the forehead 234 (at the inlet end and at the outlet end of body 12) to thereby prevent rotation of the end pieces with respect to the body 12.

As further seen in FIG. 8, the discharge port 86 is fitted with a removable discharge spout 270 which is either pressure fit or secured or by bolts, screws etc., and to which holes or other ducting means may be fitted to drain discharge water.

Attention is now directed to FIG. 10 of the drawings directed to piston body 150 and how the seal ring 162 is attached thereto. Whilst it would normally be sufficient to provide a resilient seal ring and tightly received within a circumferential grove 219 of the piston body, it is not suitable for a device according to the present invention owing to water entering the interstice between the seal ring and the groove. Such water is at low velocity (as compared with water velocity at the outer periphery 221 of the seal ring 162 and thus the pressure at the inner periphery 223 is higher than that residing at the outer periphery 221, resulting in that the seal ring 162 would tend to pop out of the groove 219. To overcome this problem without having to provide any fixture means, e.g. tightening bolts or a clamping arrangement, the seal 162 is integrally formed with sealing ribs 225 laterally projecting from its flat surfaces, for sealing engagement with the side walls of grove 219, thus sealing the interstice and preventing water ingress thereto. It is however appreciated that sealing ribs can also, or instead, be formed on the side walls of the groove 219.

While it has been shown an embodiment of the invention, it will be appreciated by a person of the art that many modifications and changes may be is made therein without departing from the spirit and the scope of the invention, mutatis mutandis. 

1. A reduced pressure zone backflow preventer comprising a body formed with an inlet port accommodating an inlet check valve assembly and an outlet port accommodating an outlet check valve assembly, said check valve assemblies extending along a coaxial axis coinciding with a longitudinal axis of the body, a central space formed between said check valves accessible through a service opening in the body, a relief valve assembly received within the central space and a drain port extending from said space; said relief assembly is responsive to pressure differential between the inlet port and the space by a flow passage extending between the inlet port and an upper side of a pressure diaphragm of the relief valve assembly; and wherein the entire body is made of plastic material.
 2. A backflow preventer according to claim 1, wherein the space accommodates a retention member fixedly receivable within the space and extending between the check valve assemblies so as to bear axial loads applied by said check valves, though without transferring axial loads between said check valve assemblies.
 3. A backflow preventer according to claim 1, wherein the inlet check valve assembly, the outlet check valve assembly, the relief valve assembly and a retention member are slidingly introduced through the service opening.
 4. A backflow preventer according to claim 1, wherein the inlet check valve assembly, the outlet check valve assembly, the relief valve assembly and a retention member are made of plastic material.
 5. A backflow preventer according to claim 1, wherein the body is made of reinforced composite plastic material.
 6. A backflow preventer according to claim 1, wherein the central space within the body has a longitudinal axis intersecting the longitudinal axis of the body at an essentially right angle.
 7. A backflow preventer according to claim 1, wherein the body assumes a shape of two cylindrical bodies intersecting at a right angle.
 8. A backflow preventer according to claim 1, wherein the inlet check valve assembly and the outlet check valve assembly are replaceable modular elements.
 9. A backflow preventer according to claim 1, wherein the inlet check valve assembly and the outlet check valve assembly are designed for foolproof assembly so as to prevent reverse assembly thereof within the body.
 10. A backflow preventer according to claim 1, wherein one or more test cocks are fitted to the body, each of which being in flow communication with a respective zone of the preventer and being selectively opened.
 11. A backflow preventer according to claim 2, wherein the retention member can be removed withdrawn for servicing the preventer even if a piston stem of the inlet check assembly is arrested at a position projecting into the central space.
 12. A backflow preventer according to claim 2, wherein the retention member is biased in its location within the central space by a spring of the relief valve assembly, whilst the retention member supports the spring and provide guidance for axial displacement of a stem member.
 13. A backflow preventer according to claim 2, wherein one of the retention member and the body is formed with at least one radially projecting rib, asymmetrically extending along the longitudinal axis thereof, for engagement with a corresponding groove formed in the other one of the retention member and the body.
 14. A backflow preventer according to claim 1, wherein the central space is fitted with a pair of parallel side walls.
 15. A backflow preventer according to claim 14, wherein the side walls are interconnected at their bottom ends via an arced surface.
 16. A backflow preventer according to claim 14, wherein the side walls constitute a double walled portion within the central space said parallel walls slidingly receive a retention member and reinforce the body.
 17. A backflow preventer according to claim 1, wherein a cylindrical portion within the body receiving the inlet check valve assembly has a first diameter at a portion adjacent the central space which is slightly larger in diameter than an end portion thereof adjacent an inlet end.
 18. A backflow preventer according to claim 1, wherein the inlet check valve assembly and the outlet check valve assembly differ from one another in the shape of their distinguishable outer casing.
 19. A backflow preventer according to claim 1, wherein the central space is formed with a normally open discharge outlet, sealable by the relief valve assembly under liquid flow through the preventer.
 20. A backflow preventer according to claim 19, wherein a plastic spout is attachable to the discharge outlet.
 21. A backflow preventer according to claim 1, wherein the inlet check valve assembly and the outlet check valve assembly are each fitted with a retraction tab or grove to facilitate extraction thereof from the body.
 22. A backflow preventer according to claim 1, wherein an inlet end and an outlet end of the body are fittable with a replaceable connecting member made of plastic material.
 23. A backflow preventer according to claim 22, wherein the connecting member is fixable to the respective inlet end and an outlet end of the body by a bracing coupler clampingly bracing a flanged rim portion of the connecting member to the body.
 24. A backflow-preventer according to claim 1, wherein the body is formed with a flow passage communicating between the inlet port and a control chamber of the relief valve assembly.
 25. A backflow preventer according to claim 24, wherein the flow passage is in flow communication with a test cock of the preventer.
 26. A backflow preventer according to claim 24, wherein the flow passage is formed by a bore extending from a flanged rim portion at an inlet end of the body, an opening of said bore being blocked by a plug.
 27. A backflow preventer according to claim 26, wherein the plug is retained in place by forehead portion of a connecting member fixedly coupled to the body.
 28. A backflow preventer according to claim 1, wherein all components of the preventer are fittable through the central space.
 29. A backflow preventer according to claim 1, wherein the inlet check valve assembly and the outlet check valve assembly are modular assemblies each comprising a cylindrical capsule slidingly accommodate a sphere-like piston body fitted with a stem member supporting a coiled spring biasing the piston member into sealing engagement against a sealing seat surface thereof.
 30. A backflow preventer according to claim 29, wherein the inlet check valve assembly and the outlet check valve assembly are packed by a cover member snappingly engaged to the cylindrical capsule.
 31. A backflow preventer according to claim 1, wherein each of the body, the inlet check valve assembly, the outlet check valve assembly, and a retention member are formed with indicia corresponding to correct assembly and flow direction of the preventer.
 32. A backflow preventer according to claim 1, wherein the relief valve assembly comprises a resilient diaphragm supported by rigid plastic disc with a stem member downwardly extending and supporting at its bottom end a sealing ring sealingly engageable with a discharge outlet of the body.
 33. A backflow preventer according to claim 32, wherein sealing ring extends above a guiding portion sized so as to ensure smooth sliding within a tubular section at the discharge outlet.
 34. A backflow preventer according to claim 32, wherein a coiled spring of the relief valve assembly bears at one end thereof against the rigid plastic disc and at an opposed end thereof against a seat of a retention member of the preventer.
 35. A backflow preventer according to claim 1, wherein the inlet check valve assembly and the outlet check valve assembly comprise a piston body fitted with a sealing ring received in a circumferential grove of the piston body and retained by sealing ribs projecting between the seal ring and side walls of the groove in a watertight fashion.
 36. A backflow preventer according to claim 35, wherein the sealing ribs are integral with the sealing ring.
 37. A backflow preventer according to claim 35, wherein the sealing ribs are integral with walls of the groove.
 38. A backflow preventer according to claim 1, devoid of any metal components apart for springs and screws. 